I found a flaw with my pump. It has a hose which I screw onto the valve stem. Nowadays, some brand new tubes have valve cores which are not screwed in tightly. I was commuting to work and got a flat, changed the tube, and every time I removed the hose, the valve core came out. I guess I'll check all my tubes before I put them in my tool bag, but I'm not happy about this. I ended up getting back on my route by flagging down a cyclist and using his pump, the kind you just push on and pull off.